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Impacts of recruitment limitation and canopy disturbance on tropical tree species richness

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Abstract

We used the process-based forest growth model Formind2.0 to show that recruitment limitation and the intermediate disturbance hypothesis which proposes maximum diversity in forests of intermediate disturbance intensity or frequency are both processes which impact on tropical tree species richness. Both processes influence each other and should therefore not be analyzed separately. While on a local level a rise in recruitment limitation promotes species richness, the overall richness of the whole forest declines. Disturbance levels are more important for species richness in forests which are highly limited by recruitment. This study supports previous field data investigations from the Americas. It also highlights the lack of consensus regarding the definition of disturbance and recruitment limitation, which makes inter-study comparisons difficult.

Introduction

One often discussed question in ecology is which processes define species diversity in the species rich tropical rain forest flora Hubbell et al., 1999, Vandermeer et al., 2000, Molino and Sabatier, 2001. Besides an intensive analysis of field data now available from large long-term rain forest plots Condit et al., 1996, Hubbell et al., 1999 quite simplistic conceptual model were used to highlight the influence of various processes and mechanisms on species abundance and diversity in the past Hubbell, 1979, Hubbell, 2001, Yu et al., 1998, Alonso and Solé, 2000, McGill, 2003, Chave et al., 2002, Bampfylde et al., 2005. Only in a few case studies were more elaborated forest growth models used to assess the dynamics of tree species diversity Liu and Ashton, 1999, Chave, 2001.

Besides classical concepts describing species richness – e.g. the number of species increases, generally with area size MacArthur and Wilson, 1967, Plotkin et al., 2000b– other theories are still a matter of debate Brokaw and Busing, 2000, Wright, 2002. For the case of tropical tree species richness, particularly the intermediate disturbance hypothesis IDH Connell, 1978, Sheil, 1999, Vandermeer et al., 2000, Molino and Sabatier, 2001, Roxburgh et al., 2004, Sheil and Burslem, 2003, Johst and Huth, 2005 and the concept of recruitment limitation Hurtt and Pacala, 1995, Hubbell et al., 1999, Chazdon et al., 1999, Hubbell, 1999 acting as the main mechanisms for maintenance of tree species diversity have been discussed within recent years.

Here, we attempt to add a few new aspects to the discussion regarding which mechanisms promote tropical tree species richness from a modeling perspective by using, for the first time, a complex process-based forest growth model. We used the model Formind2.0 which has been used to assess the impacts of tree harvesting Kammesheidt et al., 2001, Huth et al., 2004, Huth et al., 2005, Köhler and Huth, 2004 and forest fragmentation (Köhler et al., 2003) on tropical forest dynamics. Formind2.0 calculates the carbon cycle of individual trees which belong to different plant functional types. We expanded the model to distinguish all the different 468 species found at our study site, a lowland dipterocarp rain forest in Sabah, Malaysia. We believe that a detailed process-based model like Formind2.0 which does not only consider seed dispersal and establishment (Hubbell, 1979), but also succession by including processes such as competition and reasonable tree growth rates will give new insights and is a valuable tool for questioning current ecological theories.

Section snippets

Area description

Our study area is the Deramakot Forest Reserve (DFR) in Sabah (North Borneo, Malaysia, 117°30 E, 5°25 N, 130–300 masl). Deramakot has a per-humid climate typical of the inner tropics. The mean annual temperature is 27 °C with little seasonal variation. The average annual precipitation is about 3500 mm, with no pronounced dry season. The geology of Deramakot is characterized by tertiary sediments, mostly sandstone. The soils are low in nutrients and prone to erosion once left devoid of tree

Results

Our model evaluation of the species–area-relationships (Fig. 1) shows that species richness increased with area size and followed the classical relationship S=cAz with z=0.29 corresponding well to typical values for tropical rain forests around z0.25 (Plotkin et al., 2000a). The species–area-relationship of our simulations also follows the new function S=cAzekA proposed after the reanalysis of different large tropical forest plots (Plotkin et al., 2000b).

At the maximum investigated simulation

Discussion

In our model trees compete for light and space leading to the succession dynamics in the simulated forest including the creation of canopy gaps. The results of this succession process in terms of increased mortality were compared with field data van der Meer and Bongers, 1996, Köhler and Huth, 1998 and should thus mimic the basic dynamics of forest succession in a rain forest ecosystem. The IDH is to a certain extent supported by our results, although the dependence of species richness on

Acknowledgement

We thank Karin Johst for valuable comments.

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